因為火山才有我們呼吸的氧氣？

 原文網址：https://www.u-tokyo.ac.jp/focus/en/press/z0508_00396.html

一般認為自25億年前開始，能行光合作用的微生物以相對較快的速度增加之後，地球的大氣就一直含有豐富的氧氣。而某些前導事件，又稱為「短暫氧化事件」(whiff)可能為豐富氧氣的產生打開了門路。東京大學的研究人員參與在內的團隊提出了一種機制來解釋這些事件，他們的發現提出火山活動改變環境條件的程度足以使氧化加速，而短暫氧化事件就是此過程的體現。

數十億年前的生地化循環。地質上的不同腳色，包括火山、地表之下的地函、海洋與大氣之間的複雜交互作用網絡，形成了早期生命要讓大氣氧化所需的化學混和物。圖片來源：東京大學

深吸一口氣——你曾經想過進入你肺部的空氣是什麼嗎？空氣中大部分是惰性的氮氣，而我們賴以維生的貴重氧氣只佔了21%。但是情況並非總是如此，事實上，過去有幾起大滅絕事件可以對應至空氣組成劇烈改變的時間點。而如果你回到夠久之前，就會發現早於30億年前左右的地球幾乎沒有氧氣。所以什麼地方改變了？又是如何發生？

科學家的共識認為大約距今25億年前發生了大氧化事件(Great Oxygenation Event, GOE)，原因很可能是微生物充分運用了對它們有利的環境，並在競爭不多的情況下大量繁殖。它們在本質上將大氣從富含二氧化碳轉變成了富含氧氣的狀態。由於複雜生命偏好這種含有大量氧氣的新環境，因此便接著誕生出來。但在大氧化事件之前似乎還有某些前導性的氧化事件，它們或許指出了要讓大氧化事件開始，究竟需要環境條件出現什麼樣的變化以及確切的發生時間。

「在大氣氧氣的演變過程中，海洋微生物的活動具有關鍵性的地位。然而，我們認為微生物的出現並沒有立刻導致大氣氧化，因為當時海洋裡磷酸鹽之類的養分供應有限，因此限制了藍綠菌這種可以行光合作用的微生物的活動，」東京大學地球與行星科學系的田近英一教授表示。「藍綠菌變得活躍可能需要某些重大地質事件來帶給海洋養分，比方說陸地體積成長，另一個可能性則是我們在論文中所提出的，當時應該有的強烈火山活動。」

田近英一與團隊利用數值模型模擬了地球地質歷史當中，太古宙晚期(距今30億至25億年前)地球生物、地質與化學的重要性質有什麼樣的變化。他們發現大規模的火山活動增加了大氣二氧化碳，使得氣候變暖並帶給海洋更多養分；海洋生物因此獲得更多食物，進而短暫增加大氣裡的氧氣。不過這些新增的氧氣並沒有很穩定，可能出現一下子之後便消失了，因此成為了我們看到的短暫氧化事件。

「瞭解這些短暫氧化事件對於界定光合作用微生物的出現時間來說非常重要。目前推論這些事件曾經發生的方式，是透過地質紀錄中對於大氣氧含量敏感的元素濃度，」訪問研究學者渡邊泰士表示。「研究最大的困難是要建構出一個數值模型，可以模擬在太古宙晚期的環境條件下，行為複雜多變的生地化循環是如何進行。我們基於使用類似模型來模擬其他時期與目的時所得到的共同經驗，優化之後再將各個元件結合起來，才成功模擬了太古宙發生猛烈的火山爆發事件之後，地球系統會有什麼樣的複雜行為。」

 

Are volcanoes behind the oxygen we breathe?

It is widely believed that Earth’s atmosphere has been rich in oxygen for about 2.5 billion years due to a relatively rapid increase in microorganisms capable of performing photosynthesis. Researchers, including those from the University of Tokyo, provide a mechanism to explain precursor oxygenation events, or “whiffs,” which may have opened the door for this to occur. Their findings suggest volcanic activity altered conditions enough to accelerate oxygenation, and the whiffs are an indication of this taking place.

Take a deep breath. Do you ever think about the air entering your lungs? It’s mostly inert nitrogen, and the valuable oxygen our lives depend on only accounts for 21%. But this hasn’t always been the case; in fact, several mass extinction events correspond to times when this figure changed dramatically. And if you go back far enough, you’ll find that before about 3 billion years ago, there was hardly any oxygen at all. So what changed, and how did it happen?

The scientific consensus is that about 2.5 billion years ago, the Great Oxygenation Event (GOE) took place, most likely due to a proliferation of microorganisms exploiting favorable conditions and facing little competition. They would have essentially converted the carbon dioxide-rich atmosphere into an oxygen-rich one, and following that came complex life, which favored this new abundance of oxygen. But it seems there were some precursor oxygenation events prior to the GOE that may indicate the exact nature and timing of changes in the conditions necessary for the GOE to begin.

“Activity of microorganisms in the ocean played a central role in the evolution of atmospheric oxygen. However, we think this would not have immediately led to atmospheric oxygenation because the amount of nutrients such as phosphate in the ocean at that time was limited, restricting activity of cyanobacteria, a group of bacteria capable of photosynthesis,” said Professor Eiichi Tajika from the Department of Earth and Planetary Science at the University of Tokyo. “It likely took some massive geological events to seed the oceans with nutrients, including the growth of the continents and, as we suggest in our paper, intense volcanic activity, which we know to have occurred.”

Tajika and his team used a numerical model to simulate key aspects of biological, geological and chemical changes during the late Archean eon (3.0-2.5 billion years ago) of Earth’s geologic history. They found that large-scale volcanic activity increased atmospheric carbon dioxide, thereby warming the climate, and increased nutrient supply to the ocean, thus feeding marine life, which in turn temporarily increased atmospheric oxygen. The increase in oxygen was not very steady, though, and came and went in bursts now known as whiffs.

“Understanding the whiffs is critical for constraining the timing of the emergence of photosynthetic microorganisms. The occurrences are inferred from concentrations of elements sensitive to atmospheric oxygen levels in the geologic record,” said visiting research associate Yasuto Watanabe. “The biggest challenge was to develop a numerical model that could simulate the complex, dynamic behavior of biogeochemical cycles under late Archean conditions. We built upon our shared experience with using similar models for other times and purposes, refining and coupling different components together to simulate the dynamic behavior of the late-Archean Earth system in the aftermath of the volatile volcanic events.”

原始論文：Yasuto Watanabe, Kazumi Ozaki, Mariko Harada, Hironao Matsumoto, Eiichi Tajika. Mechanistic links between intense volcanism and the transient oxygenation of the Archean atmosphere. Communications Earth & Environment, 2025; 6 (1) DOI: 10.1038/s43247-025-02090-x

引用自：University of Tokyo. "Are volcanoes behind the oxygen we breathe?."